1. Field of the Invention
The present invention relates to printing methods, printing apparatuses, and computer-readable storage media.
2. Description of the Related Art
Straight-feed printers (in which a medium is carried straightly) and drum-feed printers (in which a medium is carried while being bore on a drum) are known as printing apparatuses that perform printing by moving (or “scanning”) a print head in a moving direction (or “main-scanning direction”). U.S. Pat. No. 4,198,642 and Japanese Patent Application Laid-open Publication No. 53-2040 disclose a technique, which is referred to as the “interlace scheme”, for improving the image quality of such types of printers, and in particular, inkjet printers.
FIG. 29 is a diagram for illustrating an example of the interlace scheme. It should be noted that the following parameters are used in the present specification for defining each printing scheme:                N: number of nozzles (pieces)        k: nozzle pitch (dot pitch)        s: number of times scanning is repeated        D: nozzle density (pieces/inch)        L: sub-scanning pitch (inch)        w: dot pitch (inch)        
The number of nozzles N (pieces) is the number of pieces of nozzles that are used for forming dots, and indicates the maximum number of nozzles that can be used upon one scanning movement in the main-scanning direction. In the example of FIG. 29, N=3. The nozzle pitch k (dot pitch) indicates the number of pitches of a printed image (i.e., the number of dot pitches w) that amounts to an interval between the centers of two nozzles in the print head. In the example of FIG. 29, k=2. The number of times scanning is repeated s (times) indicates the number of times of main-scanning movements required for filling up one main-scan line with dots. In the example of FIG. 29, each main-scan line is filled up with one main-scanning movement, and therefore, s=1. As described in detail below, if s is two or more, then dots will be formed intermittently in the main-scanning direction. The nozzle density D (pieces/inch) indicates the number of nozzles arranged per inch in a nozzle array of the print head. The sub-scanning pitch L (inch) indicates the distance over which a medium is moved per one sub-scanning movement. The dot pitch w (inch) indicates the pitch between dots in a printed image. It should be noted that generally, w=1/(D·k) and k=1/(D·w) hold true.
In FIG. 29, the circles, each containing a two-digit number, indicate the positions at which dots are printed. As indicated by the legend shown in FIG. 29, the number on the left, of the two-digit number in one circle, indicates the nozzle number, and the number on the right indicates the printing order (i.e., the number of the main-scanning movement during which that dot was printed).
The interlace scheme shown in FIG. 29 features the nozzle array configuration in the print head and the way in which sub-scanning movement is performed. More specifically, according to the interlace scheme, the nozzle pitch k, which indicates the interval between the centers of two adjacent nozzles, is set to be an integer of two or more, and coprime integers are selected as the number of nozzles N and the nozzle pitch k. Further, the sub-scanning pitch L is set to N/(D·k) (=N·w).
The interlace scheme is advantageous in that it is possible to disperse, over the printed image, variations in nozzle pitch, ink ejection characteristics, and so forth. Therefore, even if there are variations in nozzle pitch and/or ejection characteristics, the interlace scheme has the effect of being able to lessen the influence caused by such variations, thus improving image quality.
Japanese Patent Application Laid-open Publication No. 3-207665 and Japanese Patent Application Examined Publication No. 4-19030 disclose another technique, which is referred to as the “overlapping scheme” or the “multi-scan scheme”, aimed at improving the image quality of color inkjet printers.
FIG. 30 is a diagram for illustrating an example of the overlapping scheme. In the overlapping scheme of this example, eight nozzles are divided into two nozzle groups. The first nozzle group is made up of the four nozzles whose nozzle numbers (i.e., the numbers on the left in each circle) are even, and the second nozzle group is made up of the four nozzles whose nozzle numbers are odd. In the first main-scanning movement, dots are formed in the main-scanning direction at intervals of (s−1) dots by driving each nozzle group at intermittent timings. In the example of FIG. 30, every other dot is formed because s=2. Further, the timings for driving each nozzle group are controlled such that each group forms dots at different positions in the main-scanning direction. More specifically, as shown in FIG. 30, between the nozzles in the first nozzle group (with nozzle numbers 8, 6, 4, and 2) and the nozzles in the second nozzle group (with nozzle numbers 7, 5, 3, and 1), the printing positions are misaligned in the main-scanning direction by one dot pitch. By performing the main-scanning movements for a plurality of times and shifting the timing for driving the nozzle groups per each main-scanning movement, all dots of each main-scan line are formed.
With the overlapping scheme, the dots of a main-scan line are not printed by a single nozzle, but they are printed using several nozzles. Therefore, even if there are variations in nozzle characteristics (such as characteristics in pitch and/or ejection), it is possible to prevent such characteristics of a specific nozzle from affecting the whole main-scan line, and thus, it is possible to improve image quality.
In printers that perform printing by driving a print head in the main-scanning direction, there are situations in which “banding” (i.e., unevenness in printing that appears in band-like strips) occurs due to misalignment of the angle at which the print head is mounted.
FIG. 31 is a diagram for illustrating how banding occurs. In this example, the values for the print head 200 are set as follows: number of nozzles N=4; k=2; s=2; D=360 (dpi); and as for the sub-scanning pitch L, two kinds of values, i.e., a value that is 3/2 times the nozzle pitch k and a value that is half the nozzle pitch k, are mixed. It should be noted that the matrix-like outer border 210 shown in FIG. 31 is only for elucidating the dot forming positions.
In the example of FIG. 31, the left end of the outer border 210 is regarded as the starting position, and during the first scanning movement, four dots are formed at two-dot intervals in the sub-scanning direction, and dots are formed at two-dot intervals in the main-scanning direction. After a sub-scanning movement for a distance amounting to 3/2 times the nozzle pitch is carried out, dots are formed in the same way as described above, taking the left end of the outer border 210 as the starting position as in the first scanning movement. Then, after a sub-scanning movement for a distance amounting to half the nozzle pitch is carried out, dots are formed in the same way as described above, taking the position that is shifted from the left end of the outer border 210 towards the right by one dot as the starting position. Next, after a sub-scanning movement for a distance amounting to 3/2 times the nozzle pitch is carried out, dots are formed in the same way as described above, taking the position that is shifted from the left end of the outer border 210 towards the right by one dot as the starting position.
The matrix-like area within the outer border 210 is filled in by repeating the above-described operations.
FIG. 32 is a diagram showing how dots are formed according to the same printing method as FIG. 31, but when the print head 200 is tilted by an angle θ. As shown in FIG. 32, if the print head 200 is tilted by the angle θ, then at the upper end of the print head 200, the dots that have been formed are shift towards the left, whereas at the lower end, the dots are shifted towards the right. Thus, as shown in FIG. 33, in some positions of the dots, there appear sections 220 in which the dots are densely gathered and sections 230 in which the dots are sparsely scattered. These sections are recognized respectively as sections with high density and sections with low density compared to peripheral sections, and this causes deterioration in image quality.